In the present state of the art, three classes of tampers are used for compacting the bottoms of trenches or locations at which accessibility to vibrating plates or rollers is difficult.
The first class includes those tampers which are driven by a gasoline or diesel engine, including a rod and crank system which, by means of springs, drives a weight which is connected to a foot which strikes the ground and tamps it.
The second class includes those tampers comprising a piston sliding in a cylinder, as well as a system for injecting fuel oil, acting as a diesel engine. The design of these machines is analogous to that of the free-piston generator of Pescara, but in this case the energy supplied by the combustion of the fuel oil serves to drive the body of the machine toward the ground to compact it.
The third class of tampers includes those driven by compressed air, and including a rotary valve which ensures the distribution of the air as a function of the position of the piston in the body of the tamper, the control of the piston valve being accomplished by the sliding of fingers, connected to the valve, in helical grooves cut in the piston.
The latter machines, operating on a principle described in French Patent No. 1,396,104 in the name of one of the present inventors, as well as the previous tampers with diesel engines, have a common feature, a heavy piston which weighs approximately one-third of the weight of the machine, i.e., 15 to 50 kg.
In view of the considerable weight of this piston, it will be understood that any system which makes it possible to cause it to move in a reciprocating manner within its cylinder will likewise cause a reciprocating movement of the cylinder, in accordance with the law of action and reaction, with the compressed air or gas always simultaneously bearing against the surface of the piston and the bottom of the cylinder which it faces.
This pneumatic connection can exist for both faces of the piston, or for only one face, if the other end of the piston is elastically linked to the other end of the cylinder by means of a spring or a plurality of springs.
Since the weight-spring systems have the feature, valuable in this context, of entering into resonance if they are excited at their natural frequency, this makes it possible in the case of tampers to manufacture machines which jump higher. They are separate from the pneumatic system which is the object of the present invention.
Since it is possible to use the springs on one side of the piston, with compressed air control acting on the other side, this system is not desirable as it produces disorderly movements in the tampers which are equipped with it. The vibration which ensures compacting has superimposed upon it a slow vibration whose frequency is several cycles per minute.
At the nodes of this second vibration, the foot of the tamper jumps only slightly and at the troughs it jumps too high, thereby presenting a danger to the feet or legs of the operator controlling it, operation of the machine therefore being more or less uncontrollable.
Furthermore, such a system always introduces a source of fragility inherent in the presence of springs whose vibrations are not controlled. When the foot strikes a hard object, the shock wave is propagated from the foot into the spring wire, causing the latter to vibrate at a high frequency and with an intensity which can be disastrous.
Therefore, all tampers or other similar devices such as rams, listed above, are complex, and accordingly, costly and fragile. The juxtaposition of these two characteristics results in an increased purchase price, and especially a consumption of spare parts, and therefore prohibitive maintenance costs, resulting in a definite disaffection on the part of contractors for this class of equipment.